Bump minSdk to 21

Test: presubmit
Bug: 322508920
Bug: 329997430
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1448a452b523404d442ebd02fd5d2c0241ce8609)
Merged-In: I080e1ab31139379423e74e44ff3fa76b36becbb3
Change-Id: I080e1ab31139379423e74e44ff3fa76b36becbb3
1 file changed