[Hub Animations] Rework Hub Overview Color Management

- No longer allow for changing toolbar color alpha. Instead we will
	only allow for changing hub overview color alpha
- HubColorMixer now manages alpha value mixing as well
- Renames some methods/members
- Fixes tablet status bar color blend (earlier, the status bar would
        not transition colors, and immediately change colors)
- Before this CL: https://drive.google.com/file/d/1U1aBw8-mlI5JPyJaB5gVMdz2w2L7GuTw/view?usp=drive_link&resourcekey=0-poF8KcXgU_MErCw7kf_XRg
- After this CL: https://drive.google.com/file/d/1XRZaJ3W4K_DtbUePT0ln6KKo1icZjNnN/view?usp=sharing&resourcekey=0-mDLQ4RODMIFGI1wBS9USHg

Bug: 395106107
Change-Id: I2a3d7f89dc15a8dce865f012d24d5c3bb0b12cb5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6395700
Reviewed-by: Sky Malice <skym@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Fiaz Muhammad <mfiaz@google.com>
Cr-Commit-Position: refs/heads/main@{#1444341}