Don't crash skiagl backend.

The skiagl backend was incorrectly being created as skiaglthreaded,
which breaks mapping ExternalTextures off-thread. Specifically, there is
a workaround that disables mapping ExternalTextures at all when the
skiagl backend is used. Don't break that workaround by propagating the
renderengine type properly to implementations.

Bug: 205591213
Bug: 196334700
Bug: 193212592
Test: `adb shell stop; adb shell setprop debug.renderengine.backend
skiagl; adb shell start` boots

Change-Id: I455d5d613ccaa210dc748969e025dc86c78080b8
Merged-In: I455d5d613ccaa210dc748969e025dc86c78080b8