SDK / exoplayer

Main improvement is to switch away from ExoPlayerTestRunner and to
split out the waiting component so that the same class can be used
from Robolectric tests in the future.

The only functional difference is that that BufferLengthLogAction
is removed completely because the data isn't used downstream and the
method of collecting this data is a bit arbitrary.

PiperOrigin-RevId: 355117850