SDK / exoplayer

This is a preliminary step toward adding a DataSpec.Builder,
which is needed for sanity when adding DataSpec.customData.

The existing absoluteStreamPosition field is very error prone,
because anyone using a Builder to adjust the request position
will need to remember to adjust two values:

dataSpec
    .buildUpon()
    .setAbsoluteStreamPosition(x)
    .setPosition(x)
    .build();

Furthermore, the difference between position and
absoluteStreamPosition is irrelevant in nearly all cases. In
the core library, the difference is only relevant when initializing
AES encryption/decryption to write/read cache files.

Replacing absoluteStreamPosition with uriPositionOffset will
simplify the code block above to:

dataSpec
    .buildUpon()
    .setPosition(x)
    .build();

PiperOrigin-RevId: 294485644