Fix Level and BitDepth in AV1 Codec String

[saisrirammortha]

#917

The logic of generating codec string for AV1 had a bug. It is giving same Codec String for all the generated manifests. Bitdepth is added as a constant value

For Bitdepth
From AV1 Spec (https://aomediacodec.github.io/av1-spec/av1-spec.pdf), Section 5.5.2. Color config syntax

We can derive this statements for Bitdepth

If high_bitdepth is 0, then the value for bitdepth is 08.
If high_bitdepth is 1 and twelve_bit is 0, then the value for bitdepth is 10.
If high_bitdepth is 1 and twelve_bit is 1, then the value for bitdepth is 12.

For Level
From AV1 Spec (https://aomediacodec.github.io/av1-spec/av1-spec.pdf), Section Annex A: Profiles and levels, A.3. Levels
Seq Level Index value can be stored for the Level. We need not right shift Seq Level Index by 4 bits
This 2 digit Seq Level Index can be converted to the X.Y format level format, where X = 2 + (seq_level_index >> 2) and Y = seq_level_index & 3

[saisrirammortha]

@barbibulle Please check this when you find time.

[saisrirammortha]

We can avoid adding additional details in the Codec String as those values are default. This is mentioned in the spec

https://aomediacodec.github.io/av1-isobmff/#codecsparam

What do you suggest?